Study On MAC Protocol In UWB Wireless Network

IntroductionUltra-wideband (UWB) transmission is an emerging wireless communication technology with unique merits such as high-rate, low-transmission power, immunity to multipath propagation, and capability in precise positioning. It has received significant interests for future wireless communications from both academia and industry. In UWB wireless networks, medium access control (MAC) is essential to coordinate the channel access among competing devices. Medium access control plays a very important role in UWB wireless networks to support effective and efficient communications. The unique UWB characteristics not only pose significant challengies but also offer great opportunities in efficient UWB MAC design.More flexibility can be obtained from the inherent support of simultaneous transmissions in UWB technologies, but it also leads to a relatively complex MAC protocol in terms of power and rate allocation, interference control, and fairness mechanism in a distributed manner. The cross-layer approach should be exploited in UWB system design for better performance.Study reasonMAC design is a very challenging task in UWB networks due to the following reasons.First of all, UWB networks have a very string transmission power constraint for coexistence with other narrowband networks. Very low-transmission power is also important for non-cooperative UWB networks, which may operate simultaneously at a close range. The low-power requirement puts significance on power control, while provides opportunities for supporting simultaneous transmissions as long as the communication pairs are separated far enough in space.On the other hand, UWB physical layer characteristics also provide new opportunities for designing an effective and efficient MAC protocol. For example, its large bandwidth and low-transmission power allow the feasibility of exclusion region concept, its unique pulse transmission provides more flexibility in resource allocation, and its inherent capability in positioning simplifies routing and power control. Taking advantages of all these opportunities facilitate effective and efficient MAC.Our findingOur main finding can be summarized in the following four items.1) Sending at the Full Power: When data is sent over a link, the sender should transmit with the maximum allowed power. Otherwise, the link should remain silent. The reason is that the rate on a UWB link can always increase when the SNR at receive increases, even this increase is small. Even though an increase of a transmission power will increase interference at other nodes sending at the same time, we find that this will always be compensated by the increase of the rate on the link itself.This is again in contrast with 802.11, where a source can use only one of a few modulation, and hence the rate, a priori based on the link conditions. It is then shown in the paper that a sender should send datas with the minimal power necessary to reach the SNR threshold of the receiver, in order to decrease the interference on others and maximize performance.2) Minimum Energy and Loss Routing: The routing that maximizes total rate utility in a UWB networks within the routes considered is the MELR itself. In addition, if maximum power constraints are uniform, and nodes are not mobile, this optimal route coincides with what is usually called the minimum energy route. In the paper, we find that MELR makes UWB wireless networks utilities better than DIR3) Exclusion Regions: Although a link can adapt the rate to an arbitrary level of interference at the receiver, this might not be optimal from the rate viewpoint. It is optimal to have an exclusion regions around the destination of an active link, when acting nodes are lot. All nodes in the exclusion region have to remain silent. However, nodes outside of the exclusion region can transmit data in parallel. The destination will, thus, experience interference only from nodes outside of the exclusion region and will adapt the rate accordingly. This is very much in contrast to the 802.11 CSMA/CA mechanism, where an exclusion region is defined around the source and the destination of a link at the same time, and where the size of the exclusion region is adapted such that the interference at the receiver is smaller than a threshold, while the rate is kept fixed. But, we also find an exclusion region is not required, when the number of the acting node is small.Conclusions and Future WorkWe have answered questions on what objective the MAC layer and the routing protocol should have in a multihop UWB ad hoc network. We have presented a general model for joint scheduling, power allocation, and routing optimization problem. We have derived the exact optimal power allocation for an arbitrary network, for any scheduling and routing optimization problem for symmetric 1-D networks, and approximately bynumerical simulations for arbitrary networks in a plane. Based on these solutions, we have identified findings that hold quite generally in arbitrary networks.These findings give directions for implementations of routing and MAC protocol. A routing protocol should be based on a distributed shortest path algorithm considering inverse links attenuations as the costs of links. Although we have not directly discussed the issues of QoS in UWB networks in the sense of rate of power guarantees, our work gives important guidelines for design of QoS aware protocols. The maximization of rates in QoS network is a fundamental concept, as well as fairness. The actual integration of these concepts in a QoS scheme is for future work.